IN a New Yorker cartoon that appeared in 1993, one cyberspace-cruising dog tells his canine pal, "On the Internet, nobody knows you're a dog." The point, as anyone who has spent much time online knows all too well, is that the medium is unusually conducive to deception.

It is a vulnerability that seriously ill people should bear in mind as they seek out the medical information and sense of community offered by online patient support groups.

Cautionary tales about online liars—from sexual predators who target youngsters to con artists who peddle get-rich-quick schemes—are familiar to those who use online networks. But while deception revolving around sex and money surprises few experienced cybersurfers, unsuspecting people who frequent online medical support groups are finding themselves hoodwinked by individuals who claim they are desperately ill.